1. Technical Field
The present invention relates to carbon foam for use in batteries. More particularly, the present invention relates to low conductivity carbon foams having improved cell size uniformity and high pore volume, providing for a better lead-acid battery. In addition, the inventive carbon foams have better chemical resistance than graphitic foams and are thus less likely to intercalate, swell, change shape, etc. The invention also includes the cell components of a battery which incorporate low conductivity carbon foam.
2. Background Art
Typical lead acid batteries are the most commonly utilized rechargeable batteries, comprising generally at least one positive element, at least one negative element, and an electrolytic solution. One of the drawbacks of lead acid batteries is that both the positive elements and the negative elements are formed from lead, thereby giving the battery a substantial weight. However, these batteries have maintained popularity as they are relatively low cost and can provide ample power for items such as starter motors for automobiles despite having one of the lowest energy-to-weight ratios of any currently produced battery type.
As noted, the substantial weight of a lead acid battery is due mostly to the lead elements which comprise the positive and negative elements within the battery. These elements function in transferring a current to and from terminals of a battery during both the charging and discharging of the battery. This action is facilitated by a paste within the battery, typically a lead paste, which provides for a chemical reaction which either stores or expends energy as an electric current from the battery.
The standard lead acid battery for an automobile is designed for a 12-volt system, most often including six cells of up to 2.1 volts. The cells contains positive and negative lead elements which function as electrodes, including both lead metal and also lead dioxide. The positive lead electrode in a charged state has paste containing lead dioxide whereas the negative lead electrode can be material such as lead in a sponge formation. The electrolytic solution is most often 6-12 molar sulfuric acid which encompasses both the positive and negative electrodes.
An inherent problem with traditional lead acid batteries is that during the battery's functional life, the lead dioxide is slowly converted into lead sulfate. Lead sulfate is considered a corrosion product and can impede the transfer of electrical energy to the positive electrode. An additional problem of this corrosion is the mechanical effects of the accumulation of the corrosion within the battery. Specifically, lead sulfate will cover the layer of the lead dioxide present on the positive electrode and can continuously compound resulting in a significant volume expansion within the battery, as the lead sulfate corrosion is less dense than other elements of the battery.
This volume expansion within the battery can result in a much less than desired electrical performance as well as poor recharge characteristics. Even more problematic, the volume expansion can impart significant mechanical stress upon the positive electrode resulting in electrode deformation as well as potential cell failure within the battery. As such, the performance of the battery will substantially decrease causing a decrease in the desired service life of the battery.
As a result of the corrosion problem associated with lead acid batteries, various attempts have been made to improve the performance of batteries. One method of improving the durability of a lead acid battery is to provide the positive electrode with improved corrosion resistance. One proposed method of accomplishing this is by using a carbon material for the composition of the positive electrode, as carbon is much less susceptible to oxidation and the resulting corrosion than is lead under the same operating conditions. One such method is described in U.S. Pat. No. 5,512,390 to Obushenko. The Obushenko patent claims this battery is advantageous over traditional lead acid batteries as graphite plates may serve as both positive and negative electrodes resulting in considerable weight savings when compared to a conventional lead acid battery. Furthermore, the battery of the Obushenko patent assertedly does not experience the corrosion associated with the lead electrodes of conventional lead acid batteries.
While the use of carbon instead of lead in a lead acid battery can alleviate many of the problems associated with conventional lead acid batteries, there are problems with specific modes of carbon used within the batteries. The Obushenko patent suffers from the inherent problems associated with graphite, graphite being a relatively flat and dense material with little surface area available for the necessary amount of paste to create the desired electrical performance. Essentially, an increase in the surface area of a carbon element would provide for more energy transfer thus providing for greater electrical discharge and recharge characteristics.
An alternative carbon structure for replacing the lead plates in a lead acid battery is carbon foam.
Carbon foams have attracted considerable interest recently because of their properties of low density, coupled with either very high or low thermal conductivity. Conventionally, carbon foams are prepared by two general routes. Highly graphitizable foams have been produced by thermal treatment of mesophase pitches under high pressure. These foams tend to have high thermal and electrical conductivities. For example, in Klett, U.S. Pat. No. 6,033,506, mesophase pitch is heated while subjected to a pressure of 1000 psi to produce an open-cell foam containing interconnected pores with a size range of 90-200 microns. According to Klett, after heat treatment to 2800° C., the solid portion of the foam develops into a highly crystalline graphitic structure with an interlayer spacing of 0.366 nm. The foam is asserted to have compressive strengths greater than previous foams (3.4 MPa or 500 psi for a density of 0.53 g/cc).
In Hardcastle et al. (U.S. Pat. No. 6,776,936) carbon foams with densities ranging from 0.678-1.5 gm/cc are produced by heating pitch in a mold at pressures up to 800 psi. The foam is alleged to be highly graphitizable and provide high thermal conductivity (250 W/mK).
According to H. J. Anderson et al. in Proceedings of the 43dInternational SAMPE Meeting, p 756 (1998), carbon foam is produced from mesophase pitch followed by oxidative thermosetting and carbonization to 900° C. The foam has an open cell structure of interconnected pores with varying shapes and with pore diameters ranging from 39 to greater than 480 microns.
Rogers et al., in Proceedings of the 45th SAMPE Conference, pg 293 (2000), describe the preparation of carbon foams from coal-based precursors by heat treatment under high pressure to give materials with densities of 0.35-0.45 g/cc with compressive strengths of 2000-3000 psi (thus a strength/density ratio of about 6000 psi/g/cc). These foams have an open-celled structure of interconnected pores with pore sizes ranging up to 1000 microns. Unlike the mesophase pitch foams described above, they are not highly graphitizable. In a recent publication, the properties of this type of foam were described (High Performance Composites September 2004, pg. 25). The foam has a compressive strength of 800 psi at a density of 0.27 g/cc or a strength to density ratio of 3000 psi/g/cc.
Stiller et al. (U.S. Pat. No. 5,888,469) describes production of carbon foam by pressure heat treatment of a hydrotreated coal extract. These materials are claimed to have high compressive strengths of 600 psi for densities of 0.2-0.4 gm/cc (strength/density ratio of from 1500-3000 psi/g/cc). It is suggested that these foams are stronger than those having a glassy carbon or vitreous nature which are not graphitizable.
Carbon foams can also be produced by direct carbonization of polymers or polymer precursor blends. Mitchell, in U.S. Pat. No. 3,302,999, discusses preparing carbon foams by heating a polyurethane polymer foam at 200-255° C. in air followed by carbonization in an inert atmosphere at 900° C. These foams have densities of 0.085-0.387 g/cc and compressive strengths of 130 to 2040 psi (ratio of strength/density of 1529-5271 psi/g/cc).
In U.S. Pat. No. 5,945,084, Droege described the preparation of open-celled carbon foams by heat treating organic gels derived from hydroxylated benzenes and aldehydes (phenolic resin precursors). The foams have densities of 0.3-0.9 g/cc and are composed of small mesopores with a size range of 2 to 50 nm.
Mercuri et al. (Proceedings of the 9th Carbon Conference, pg. 206 (1969)) prepared carbon foams by pyrolysis of phenolic resins. For foams with a density range of 0.1-0.4 g/cc, the compressive strength to density ratios were from 2380-6611 psi/(g/cc). The pores were ellipsoidal in shape with pore diameters of 25-75 microns) for a carbon foam with a density of 0.25 gm/cc.
Stankiewicz (U.S. Pat. No. 6,103,149) prepares carbon foams with a controlled aspect ratio of 0.6-1.2. The patentee points out that users often require a completely isotropic foam for superior properties with an aspect ratio of 1.0 being ideal. An open-celled carbon foam is produced by impregnation of a polyurethane foam with a carbonizing resin followed by thermal curing and carbonization. The pore aspect ratio of the original polyurethane foam is thus changed from 1.3-1.4 to 0.6-1.2.
In Kelley et al., U.S. Pat. No. 6,979,513, carbon foam is used as an electrode plate within an electrode chemical battery. The carbon foam is conductive 25 μ-ohm-m and made from a wood substrate which is carbonized to form a carbonized wood current collector.
Subsequently, the Kelley et al. patent discloses that a chemically active material may be disposed on the carbonized wood current collector to function as the electrode plate within the battery.
Unfortunately, the carbon foams that have been utilized thus far for electrode chemical batteries, specifically lead acid electrode chemical batteries, leave much room for improvement. The carbon foams generally available are not monolithic and do not have the uniformity, strength and density requirements for such applications. In addition, these foams do not have a high enough porosity or surface area making them ill suited for containment of a chemical paste. Moreover, conventional belief is that a foam material used in battery applications needs to be highly conductive (i.e., have low electrical resistance), and thus must be graphitic or graphitizable.
There is desired, therefore, a carbon foam material which has the controllable cell structure where the cell structure strength, density, and strength-to-density ratio make the foam suitable for use in electrochemical batteries as well as in other applications. Indeed, a combination of characteristics including uniformity, high strength and a lower relative density, even with an electrical resistivity of greater than about 100 μohm-meters have now been found to be useful for an improved electrochemical battery wherein carbon foam is utilized as at least one electrode plate.